The development of high energy battery systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anode materials, such as lithium, sodium and the like, and the efficient use of high energy density cathode materials, such as FeS.sub.2 and the like. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, to turn to the investigation of nonaqueous electrolyte systems and more particularly to nonaqueous organic electrolyte systems.
The term "nonaqueous organic electrolyte" in the prior art refers to an electrolyte which is composed of a solute, for example, a salt of a complex salt of Group 1-A, Group II-A or Group III-A elements of the periodic Table, dissolved in an appropriate nonaqueous organic solvent. Conventional solvents include propylene carbonate, 1-2-dimethoxyethane (DME), tetrahydrofuran, ethylene carbonate, 3-methyl-2-oxazolidone (3Me2Ox), 3,5-dimethylisoxazole (DMI) or the like.
A multitude of solutes is known and recommended for use but the selection of a suitable solvent has been particularly troublesome since many of those solvents which are used to prepare electrolytes sufficiently conductive to permit effective ion migration through the solution are reactive with the highly reactive anodes described above. Most investigators in this area, in search of suitable solvents, have concentrated on aliphatic and aromatic nitrogen- and oxygen-containing compounds. The results of this search have not been entirely satisfactory since many of the solvents investigated still could not be used effectively with extremely high energy density cathode materials and were sufficiently corrosive to lithium anodes to prevent efficient performance over any length of time.
U.S. Pat. No. 4,071,665 titled "High Energy Density Battery With Dioxolane Based Electrolyte" discloses high energy density galvanic batteries having high utilization of active electrode materials such as lithium anodes, cathode depolarizers reducible by said anodes, such as cupric sulfide, and electrolytes comprising a dioxolane as solvent and up to about 20 weight percent of a conductive non-reactive electrolyte salt such as lithium perchlorate dissolved therein. Optionally up to 50 weight percent of the solvent can be a second solvent which is an aliphatic or cycloaliphatic carbohydric ether to reduce battery gassing. Additional small amounts of a tertiary nitrogen base can be added to suppress the tendency of the electrolyte system to polymerize.
U.S. Pat. No. 4,952,330 titled "Nonaqueous Electrolyte" discloses a nonaqueous electrolyte for cells such as Li/FeS.sub.2 cells, comprising a solute, such as LiCF.sub.3 SO.sub.3, dissolved in a mixture of a major amount of dioxolane (i.e. 40-53 volume percent), a minor amount of propylene carbonate (i.e. 8-18 volume percent) and dimethoxyethane (i.e. 32-45 volume percent). The dioxolane to dimethoxyethane weight ratio for these solutions cover the range of 67:33 to 52:48.
U.S. Pat. No. 3,996,069 titled "Nonaqueous Cell Utilizing a 3Me2Ox-based Electrolyte" discloses a nonaqueous cell utilizing a highly active metal anode, such as lithium, a solid cathode selected from the group consisting of FeS.sub.2, Co.sub.3 O.sub.4, V.sub.2 O.sub.5, Pb.sub.3 O.sub.4, In.sub.2 S.sub.3 and CoS.sub.2, and a liquid organic electrolyte consisting essentially of 3-methyl-2-oxazolidone in combination with a low viscosity cosolvent, such as dioxolane, and a metal salt selected, for example, from the group consisting of MSCN, MCF.sub.3 SO.sub.3, MBF.sub.4, MClO.sub.4 and MM'F.sub.6 wherein M is lithium, sodium or potassium and M' is phosphorus, arsenic or antimony.
U.S. patent application Ser. No 744,179 filed on Aug. 13, 1991 discloses a nonaqueous electrolyte for use in electrochemical cells employing a solute such as LiCF.sub.3 SO.sub.3 dissolved in a mixture of a dioxolane-based solvent and an acyclic ether solvent, such as dimethoxyethane, in which the weight ratio of the dioxolane-based solvent to the acyclic ether solvent is less than 45:55 and the volume ratio is less than 40:60.
While the theoretical energy, i.e. the electrical energy potentially available from a selected anode-cathode couple is relatively easy to calculate, there is a need to choose a nonaqueous electrolyte for such couple that permits the actual energy produced by an assembled battery to approach the theoretical energy. The problem usually encountered is that it is practically impossible to predict in advance how well, if at all, a nonaqueous electrolyte will function with a selected couple. Although a cell must be considered as a unit having three parts--a cathode, an anode and an electrolyte --and it is to be understood that the parts of one cell are not predictably interchangeable with parts of another cell to produce an efficient and workable cell. It has now been realized that the separator, in conjunction with a specific electrolyte, can play an important part in the performance characteristics of a cell.
Many cell systems can function in various environments when they are freshly produced. However, when cell systems are stored for long periods of time at high temperatures, their impedance characteristics can deteriorate to render the cell systems unsuitable for some consumer applications. A specific harsh application of a cell is its use in cameras where flash amperages at high drain rates are required. Although cells can function under normal conditions, many cells usually exhibit high cell impedance and low amperage under high drain rates as exemplified in flash camera operations.
It is an object of the present invention to provide an electrolyte solution and separator combination for use in an electrochemical cell that can be stored at high temperatures for extended periods of time without allowing cell impedance to increase to levels which substantially reduce cell performance.
Another object of the present invention is to provide an electrolyte solution and separator combination for an electrochemical cell employing a mixture of a dioxolane-based solvent and an acyclic ether as the electrolyte and a separator having a specific pore size, porosity, resistance and tortuosity.
Another object of the present invention is to provide an electrolyte solution and separator combination ideally suited for cells employing a lithium anode and an iron sulfide-containing cathode.
Another object of the present invention is to provide an electrochemical cell employing a lithium anode, a cathode such as FeS.sub.2 and an electrolyte solution comprising a solute dissolved in a mixture of dioxolane and 1,2-dimethoxyethane and a separator having a pore size between 0.005 and 5 microns, a porosity between 30 and 70%, a resistance between 2 and 15 ohm-cm.sup.2, and a tortuosity of less than 2.5.
The foregoing and additional objects will become more fully apparent from the following description.